Has the horse been dewormed recently? Also, is he/she up to date on his/her shots? It sounds like you have tried just about every thing there is to try. Here there is a pink medicine that you can buy that is similar to pepto bismol but it works well in horses and cattle we have found. But I think it sounds like you may have already been using it. I am not sure what it is called. Maybe they may need to start an iv and offer medication interveniously if possible and fluids to see if that will help if you can afford it. I would also have another different vet look at the horse and see if they can suggest anything else that you have not tried if you don't find anything else. Just make sure you show the tests that have been done and the results so you don't have to pay any unnecessary charges you have already paid. Good luck!!!
